As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures onto a photosensitive layer, e.g., a photoresist on wafers. In order to fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, photolithographic masks will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (approximately 13.5 nm).
Photolithographic masks have to fulfil highest demands with respect to transmission homogeneity, planarity, pureness and temperature stability. For future EUV photolithographic masks the tolerable deviation of their substrates from the planarity is only a portion of a wavelength of the exposure wavelength in order to not significantly disturb the phase front of the electromagnetic wave reflected from a multi-layer structure on a surface of the substrate. Larger deviations of the planarity of the substrate of the photolithographic mask may lead to variations of the optical intensity distribution in the photoresist due to a constructive of a destructive addition of the wave front in the photoresist. At the further processing of the wafer, the variations of the optical intensity may result in the fabrication of defective semiconductor devices. Decreasing exposure wavelength makes this problem more challenging. The substrate as supplied from the manufacturer may not even fulfil the planarity condition for EUV photolithographic masks and the manufacturing process of the mask which forms fine patterns on one surface may even deteriorate the planarity of the substrate.
For transmissive photolithographic masks the homogeneity of the optical transmission across the mask area is an important parameter. A variation of the optical transmission across the area of the photolithographic mask leads to a corresponding variation of the local optical dose applied to the photoresist on the wafer. The variation of the locally applied dose results in a fluctuation or a variation of the structure dimension of a pattern element in the developed photoresist. The uniformity of a structure element across the area of the photolithographic mask is called critical dimension uniformity (CDU).
Furthermore, a curvature of the substrate of a photolithographic mask also leads to imaging errors of the mask. US patent publication 2007/0224522 A1 describes a method to improve the planarity of a manufactured photolithographic mask. To adjust a curvature of the substrate or to smooth the unevenness of the substrate, this document proposes forming a deformed or expanded portion in a predetermined region of the substrate wherein the substrate includes a curved region before forming the expanded portion. The expanded portion is generated by focussing femtosecond laser pulses in this region which locally modifies the bonding state of the substrate.
U.S. Pat. No. 7,001,697 B2 provides another method to eliminate intensity differences or optical transmission errors introduced by the photolithographic mask in the photoresist on a wafer. A diffraction pattern is etched on the rear substrate surface, which is the substrate surface opposite to the surface carrying the pattern elements, in order to compensate for the local differences in the optical intensity in the photoresist induced during a single illumination of the mask.
U.S. Pat. No. 7,241,539 B2 and US patent publication 2007/0065729 A1 disclose a further method to correct optical transmission errors or imaging errors introduced by a photolithographic mask or by the optical elements used for the illumination of the mask. By generating an array of shadowing elements in the substrate of the mask by again using femtosecond laser pulses, diffraction errors through the pattern elements are offset, so that an approximately uniform intensity of patterning radiation is transmitted through the mask substrate. Spacings, sizings and/or placements of the shadowing elements may be determined empirically using trial and error and/or by using simulation.
The action of femtosecond laser pulses on quartz or fused silica forming the substrate material of photolithographic masks has for example been investigated by S. Oshemkov, V. Dmitriev, E. Zait and G. Gen-Zvi: “DUV attenuation structures in fused silica induced ultrashort laser radiation”, Proc. CLEOE-IQEC, Munich 2007. The pending U.S. provisional patent applications 61/324,467 and 61/351,056 of the applicant, which are herein incorporated by reference in their entirety, describe some aspects of the critical dimension correction (CDC) in photolithographic masks.
In addition to errors introduced due to diffraction at the pattern elements, the pattern elements forming the photolithographic mask may also be defective. U.S. Pat. No. 7,459,242 discloses a method for repairing a photolithographic mask having also a void in the chrome layer forming the pattern elements. By introducing a diffractive optical element or a shading element (DOE/SE) in the substrate of the photolithographic mask in front of the void the scattering properties of the substrate at the position of the DOE/SE is changed, thus correcting the void in the chrome layer on the substrate of the photolithographic mask.
Moreover, photolithographic masks may also have placement errors of pattern elements, i.e. some of the pattern elements do not image the pattern parameters exactly at the predetermined position on the photoresist. The effects of placement errors of pattern elements in the photoresist are normally reduced by performing a linear imaging transformation of the photolithographic mask with respect to the focus of the image field. By a small shift of the photolithographic mask in a plane parallel to the photoresist the overall effect of placement errors can be diminished. A rotation of the mask relative to the focus of the image plane may also decrease the sum of placement errors of the pattern elements on the substrate of the photolithographic mask. Furthermore, a further possibility for correcting placement errors of pattern elements is performing a scale correction of the imaging of the pattern elements of the photolithographic mask in the photoresist on the wafer. In case the placement errors of the pattern elements are still too large after a linear imaging transformation the mask has to be discarded.
The document DE 10 2006 054 820 A1 still goes a step further. This document proposes to introduce an array of local density variation in a portion of the substrate of the mask close to the placement error in order to shift the respective pattern elements. This shift of the pattern elements in a direction in order to minimize imaging errors of the photolithographic mask on the photoresist again reduces the sum of the overall placement errors of the mask and thus increases the yield of the mask fabrication process. The local density variations in the mask substrate are generated by locally and temporarily melting the substrate material using a femtosecond laser beam. This process locally diminishes the substrate density at the range the material has been temporary melted. A dot locally changed by a laser beam is called pixel. The shape, the density and the configuration of pixels necessary to perform a desired correction of a placement error for a pattern element is determined experimentally by executing a respective illumination of a plurality of samples which have pattern elements arranged thereon. The experimental results are then stored in a library.
The document DE 10 2006 054 820 A1 describes a method in which the placement errors of pattern elements on a substrate of a photolithographic mask is reduced in a two stage process. In a first step a linear imaging transformation is performed. In a second step the remaining placement errors are further reduced by selectively introducing a density variation in the substrate of the mask. If necessary, this loop can be repeated. However, this approach still retains an amount of errors of a photolithographic mask which can in many cases not be tolerated. Moreover, for each substrate material a comprehensive library of correcting tools has to be experimentally determined prior being able to correct placement errors which are not correctable by a linear imaging transformation of the photolithographic mask.
It is therefore one object of the present invention to provide a method and an apparatus for correcting photolithographic masks, so that the error remaining after error correction is minimal and thus increasing the yield of fabricated photolithographic masks.